The M88 has more bottom than the M69 and the hi end is a tad more silky.
Both the M69 and M88 are very rejective, so in a 'recording band at once' situation, these are probably one of the best mics to use the more because they have a nice off axis response.
But a M88 on sax? I used a M88 on sousaphone lately, together with a Senny MD441. The 441 was much smoother, the 88 has too much 'crack' for that.
I've never experienced a bad Beyer mic though.